The structural organization of actin in the sea urchin egg cortex will be investigated, concentrating on the analysis of changes in cortical organizaton associated with fertilization and cytokinesis, and the control mechanisms that regulate the organization of the cortical cytoskeleton. The long-term goal of these studies is a more complete understanding of the role of the cortical cytoskeleton in the events of early development, including fertilization, cytokinesis and morphogenesis. Four specific aims are proposed. 1) The structural organization of the unfertilized egg cortex will be examined using rapid-freezing, freeze-fracture techniques. 2) The three-dimensional organization of actin in the fertilized egg cortex will be studied by the rapid-freeze, deep-etch method. 3) The role of cortical actin in the formation of the contractile ring will be studied by investigating: a) regional changes in cortical actin organization during cytokinesis, b) the relationship between cortical actin organization and contractile ring formation in eggs exposed to hyrostatic pressure, and c) the structurla organizaton of the contractile ring. 4) The role of cortical actin in microvillar formation will be studied by examining: a) microvillar formation and cortical actin reorganization at fertilization in eggs in which cortical granule exocytosis has been inhibited with hydrostatic pressure or NH4C1; b) the effect of inverting the sequence of Ca++ and pH changes at fertilizaton on the reorganization of the cortex; and c) the role of protein synthesis in the reformation of microvillar core filament bundles in eggs denuded of microvilli by high hydrostatic pressure. These studies will provide new information on the dynamics of cortical actin organization during early development and the role of cortical actin in fertilization and cytokinesis. In addition, they will contribute to a more complete understanding of the general mechanisms that regulate actin-based non-muscle cell motility. This information will aid in understanding the initiation and control of morphogenetic movements during normal embryonic development and the mechanism of tumor cell migration during metastasis.